“Airmail planes need to navigate the 2,629-mile-long route regardless of weather and time of day, so in 1923, Congress approved construction of a series of arrows and beacons to guide the planes. The arrows were painted bright yellow, and gas-powered beacons sat atop towers constructed on the arrows’ tails. A nearby shed stored gas to power the beacons. By the summer of 1924, these beacons stretched between Cleveland, Ohio, and Rock Springs, Wyoming. By 1929, they spanned the entire route.”